ON THE HISTORY OF OPTICS. 481 



cient in mathematical accuracy, and the author was not sufficientlj' master of 

 the science to distinguish the good from the indift'erent. 



Mr. Delaval's experiments on colours appear to show very satisfactorily, 

 that all the colouring substances, in common use, owe their tints to rays, 

 which are separated from white light, during its passage through them, and 

 not, as Newton supposed, to the reflection of a particular colour from the first 

 surface. It has been observed that Kepler and Zucchius had long ago made 

 experiments nearly similar to those of Mr. Delaval. Dr. Robert Darwin's 

 investigation of the eti'ects of strong lights on the eye appears to comprehend 

 almost all possible varieties of these ocular spectra, but it does not lead to 

 any fundamental analogy, capable of explaining the most intricate of them. 



The phenomena of the unusual atmospheric refraction, which frequently 

 produces double or triple images of objects seen near a heated surface, have 

 been successively illustrated by Mr. Huddart, Mr. Vince,and Dr. Wollaston, 

 so that at present there appears to be little doubt remaining with respect to 

 their origin. Dr. WoUaston's instrument, for the measurement of refractive 

 densities, very much facilitates the examination of the optical properties of sub- 

 stances of various kinds : he has applied it very successfully to the confirmation 

 of Huygens's theory of double refraction; he has corrected the common 

 opinion respecting the division of the prismatic spectrum; he discovered, 

 without being acquainted with the observations of Hitter, the dark rays which 

 blacken the salts of silver ; and he has remarked a singular property in some 

 natural as well as artificial crystals, which appear of one colour when viewed in 

 the direction of the axis, and of another when in a transverse direction. 



To Dr. Herschel the sciences of optics and astronomy are equally indebted. 

 He has carried the construction of the reflecting telescope to a degree of per- 

 fection, far exceeding all that had been before attempted, and the well known 

 improvements, which astronomy has derived from his observations, are nume- 

 rous and important. In the course of his researches for the attainment of 

 his more immediate objects, he has also had the good fortune to discover the 

 separation of the rays of heat from those of light by means of refraction; a 

 fact which has been sufficiently established by the experiments of several 

 other persons. 



VOL. I. Sq 



